1. Field of the Invention
The invention relates to a rewritable optical recording medium suitable for high-speed recording corresponding to 6× speed to 12× speed of DVD.
2. Description of the Related Art
As one of optical recording media capable of rewriting information by means of semiconductor laser beam irradiation, a so-called phase-change optical recording medium is known that utilizes phase transition between crystalline-amorphous states or between crystalline-crystalline states. This phase-change optical recording medium enables repeated recording of information by a single beam, and also has a feature such that an optical system of drive is simpler. The phase-change optical recording medium is applied to optical recording media to store data on computers, or audio and visual data and is widely utilized worldwide. CD-R, CD-RW, etc. are widely utilized, and at the same time high-speed recording is already possible. Likewise, with respect to the phase-change optical recording medium, the increase of the capacity and density of medium, thus high-speed recording is essential from the perspective of high-density image recording.
Such phase-change optical recording medium, in general, comprises a substrate and recording layer, wherein heat-resistant and translucent protective layers are disposed on the both surfaces of the recording layer. Further, a reflective layer of e.g. metal or alloy is disposed on the protective layer in the side opposite to the incident direction of light beam. Information can be recorded and erased only by changing the power of a laser beam. In general, recording is conducted as follows. Crystalline state is set as unrecorded or erased state, and recording marks are formed by heating a recording layer to the temperature higher than melting point, and then very rapidly cooling, making the recording layer amorphous state.
The recording principle of phase-change optical recording medium is as follows. The crystalline state/amorphous state of the recording layer is switched using a focused laser beam pulsed at three output levels. During the switching, highest output level is used for melting of the recording layer, medium output level is used for heating the recording layer to the temperature higher than crystallization temperature, immediately below melting point, and the lowest output level is used for controlling of heating or cooing of the recording layer. The material in the recording layer is melted by the laser pulse of the highest output level, followed by very rapid cooling, and becomes an amorphous or microcrystalline state, resulting in the reduction of reflectance of the recording layer, to form a recording mark. In the case of laser pulse with medium output, the material in the recording layer becomes a crystalline state, by which information can be erased. In this way, by using the write laser pulse with different output levels, crystalline region and amorphous region can be made alternately in the recording layer, and thus information is memorized.
In order to achieve high-speed recording, it is required to use a phase-change material of fast crystallization speed in a recording layer. As such a phase-change material, attention has been paid to Ge—Te, Ge—Te—Se, In—Sb, Ga—Sb, Ge—Sb—Te, Ag—In—Sb—Te, etc. since those have fast crystallization speed and a high erasing ratio at the time of high-speed recording.
Among these, GaSb alloy has a high crystallization temperature of 350° C. and gives a mark excellent in stability (storage reliability) (see, “Phase-Change optical data storage in GaSb”, Applied Optics, Vol. 26, No. 22115, November, 1987). Such GaSb alloy is one of phase-change materials to which the present inventors also have paid attention as the material of recording layer for high-speed recording so far.
Previous investigations by the present inventors, however, revealed that it is difficult to achieve high-speed recording corresponding to 6× speed to 12× speed of DVD using the GaSb binary alloy. Specifically, the limit velocity for crystallization (refer to explanation described later) of GaSb binary alloy is not sufficient for high-speed recording corresponding to 6× speed to 12× speed of DVD. Further, it was revealed that high crystallization temperature of GaSb makes initialization difficult, and even if initialization is performed by irradiating a recording layer with a laser beam having high power for initialization, the reflectance after initialization varies, resulting in insufficient recording properties. Further, it was revealed that even GaSb of eutectic composition has a relatively high melting point of 630° C., thus, making it difficult to form a mark and inviting insufficient sensitivity, and besides, it was revealed that defects are caused such as a low disc reflectance.
Further investigations by the present inventors led to the findings that Sn makes the crystallization speed of GaSb faster and effectively brings about lowering of melting point (thus, improvement of sensitivity), improvement of reflectance, and reduction in noise after initialization. Namely, it was found that GaSbSn ternary alloy enables high-speed/high-sensitivity recording corresponding to 6× speed to 12× speed of DVD.
While it was revealed that Sn makes the transition linear velocity of GaSb binary alloy which serves as the base of Sn faster and is an optimum additive element for high-speed recording, it was found that the addition of Sn causes a new problem that disc reflectance is reduced after 200 hours at 80° C., 85% RH (hereinafter, referring to as an “environmental test”).
In addition to the technique mentioned above, as a known technique before this application, U.S. Pat. No. 4,818,666 discloses the use of recording material containing an alloy of GaSb or InSb having a composition ratio near 50:50 to which a metal or chalcogenide element M is added, but the composition ranges of Ga and additive element M are different from those in the present invention.
Japanese Patent Application Laid-Open No. 61-168145 discloses a phase-change optical recording medium which uses as a recording material an alloy containing GaSb as a main constituent and which records information utilizing phase-change between crystalline-crystalline states. This Literature includes an example of GaSbSn, intended for increase of contrast, but did not mention the composition ratio of Sn. With respect to Ga ratio, this Literature includes a description that in the case where Ga is less than 20%, convex portion is formed at the portion irradiated with laser beam possibly attributed to the generation of air bubble, resulting in unstable variation level of reflectance, which causes problem in practical use.
Here, the limit velocity for crystallization will be described.
Limit velocity for crystallization is a physical value representing property of the material of recording layer, which the present inventors created based on experiences. A rotating optical disc is irradiated with DC light with a given power, and the dependence of reflectance of optical disc on linear velocity of irradiation light beam, i.e., rotating speed of the optical disc (note that linear velocity dependence in a recording/reproducing system) is evaluated. The limit velocity for crystallization means the linear velocity at the time when drastic reduction of the reflectance as shown in FIG. 3 starts. In this evaluation method, “DC light with a given power” is regarded as the laser pulse with medium output (erase pulse) in the description of recording principle mentioned above, evaluating to which linear velocity crystallization (erasing) is possible when the linear velocity of irradiation light beam in the recording/reproducing system is increased. Taken up FIG. 3 as an example, it shows that when the optical disc is irradiated with the DC light at a linear velocity exceeding the limit velocity for crystallization (heavy line in the figure) of the material of recording layer, the recording layer is not satisfactorily crystallized.